Ferritic stainless steel having excellent strength and corrosion resistance to acid and method of manufacturing the same

ABSTRACT

Provided are a ferritic stainless steel having an excellent strength and corrosion resistance to acid and a method of manufacturing the same. The ferritic stainless steel according to an embodiment of the present disclosure includes, by weight %, 0.1% to 0.2% of carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome, 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe) and other inevitable impurities, wherein a number of carbides having a diameter of 100 nm or more per unit area is 50 ea/100 μm2 to 200 ea/100 μm2.

TECHNICAL FIELD

The present disclosure relates to a ferritic stainless steel and a method of manufacturing the same, and more particularly, a ferritic stainless steel having an excellent strength and corrosion resistance to acid and a method of manufacturing the same.

BACKGROUND ART

A ferritic stainless steel among stainless steels is widely used for building materials, kitchen containers, home appliances, parts of vehicle exhaust system, etc.

The ferritic stainless steel has recently been applied to automotive battery cells. Automakers are demanding a higher strength and corrosion resistance than conventional ferritic stainless steels to secure long-term battery performance, and are demanding lower cost materials to lower the price of batteries.

Methods of increasing the strength of the ferritic stainless steels to meet the automakers' demands include work hardening, solid solution strengthening, precipitation hardening, and the like. However, due to the characteristics of ferritic stainless steels without phase transformation, there is a problem that the workability is drastically lowered during work hardening. Also, it is difficult to utilize Mo and Nb, which are excellent in solid solution strengthening, because they are expensive components.

Conventionally, carbon (C), which is a component damaging the workability of ferritic stainless steels, has been limited to 0.02 weight % or lower. However, when a large amount of C is added, the strength of the ferritic stainless steel can be improved due to the precipitation of carbides, and both a strength and workability can be secured when a certain degree of ductility is secured, due to recent development of processing technology.

However, in the case in which hot rolling is performed at a high temperature, a reduction ratio is low, and a coiling temperature is high even when a large amount of C is added, carbides are precipitated not finely but coarsely in the deformed structure. As a result, there is a problem that it is difficult to refine the crystal grains and to secure a desired strength.

(Patent Document 0001) Japan Patent Application Publication No. 2006-183081

DISCLOSURE Technical Problem

Embodiments of the present disclosure are directed to providing a ferritic stainless steel having an excellent strength and acid resistance by controlling alloy components of the ferritic stainless steel to control precipitates and crystal grains of the ferritic stainless steel.

In addition, embodiments of the present disclosure are directed to providing a method of manufacturing a ferritic stainless steel having an excellent strength and acidacid resistance by controlling a slab reheating temperature, a reduction ratio, and a coiling temperature during hot rolling to control precipitates and crystal grains.

Technical Solution

A ferritic stainless steel having an excellent strength and acid resistance according to an embodiment of the present disclosure includes, by weight %, 0.1% to 0.2% of carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome (Cr), 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe), and other inevitable impurities, wherein a number of carbides having a diameter of 100 nm or more per unit area is 50 ea/100 μm² to 200 ea/100 μm².

In addition, according to an embodiment of the present disclosure, an average crystal grain diameter may be 10 or less.

In addition, according to an embodiment of the present disclosure, a tensile strength may be 520 MPa or more.

In addition, according to an embodiment of the present disclosure, an elongation may be 20% or more.

In addition, according to an embodiment of the present disclosure, a critical current density I_(crit) in a 5% sulfuric acid atmosphere may be 10 mA or less.

A method of manufacturing a ferritic stainless steel having an excellent strength and acid resistance according to an embodiment of the present disclosure includes hot-rolling and cold-rolling a ferritic stainless steel slab including, by weight %, 0.1% to 0.2% of Carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome (Cr), 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe), and other inevitable impurities, wherein a value of Equation (1) during the hot rolling satisfies 1,000 or less, Equation (1) being 15*RHT/R4+CT, where RHT (° C.) represents a slab reheating temperature, R4(%) represents a reduction ratio of R4 stand of rough rolling, and CT (° C.) represents a coiling temperature.

In addition, according to an embodiment of the present disclosure, the value of Equation (1) may satisfy 800 to 1,000.

In addition, according to an embodiment of the present disclosure, RHT may be below 1,250° C. R4 may be above 40%, and CT may be below 650° C.

In addition, according to an embodiment of the present disclosure, a number of carbides having a diameter of 100 nm or more per unit area of a cold-rolled plate may be 50 ea/100 μm² to 200 ea/100 μm², and an average crystal grain diameter of the cold-rolled plate may be 10 μm or less.

Advantageous Effects

According to the embodiments of the present disclosure, the strength and acid resistance of the ferritic stainless steel may be improved by controlling alloy components and hot rolling conditions to control precipitates and crystal grains.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph for explaining a correlation between hot rolling conditions of a ferritic stainless steel and a number of carbides of a cold-rolled steel plate.

FIG. 2 is a picture showing a distribution of precipitates in a ferritic stainless cold-rolled steel plate according to an embodiment of the present disclosure, taken by a transmission electron microscope (TEM).

FIG. 3 is a picture showing a distribution of precipitates in a ferritic stainless cold-rolled steel plate according to a comparative example of the present disclosure, taken by a TEM.

FIG. 4 is a graph for explaining a correlation between a number of carbides and a tensile strength of a cold-rolled steel plate made of a ferritic stainless steel.

MODES OF THE INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to sufficiently transfer the technical concepts of the disclosure to one of ordinary skill in the art. However, the disclosure is not limited to these embodiments, and may be embodied in another form. In the drawings, parts that are irrelevant to the descriptions may be not shown in order to clarify the disclosure, and also, for easy understanding, the widths, lengths, thicknesses, etc. of components are more or less exaggeratedly shown. Like numbers refer to like elements throughout this specification.

A ferritic stainless steel having an excellent strength and acid resistance, according to an exemplary embodiment of the present disclosure, may include, by weight %, 0.1% to 0.2% of carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome (Cr), 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe), and other inevitable impurities.

Carbon (C): 0.1% to 0.2%

An amount of carbon (C) may be, by weight %, 0.1% to 0.2%. When the amount of carbon (C) is less than 0.1%, an amount of austenite generated during hot-rolling may be reduced so that ferritic band structures remain without being destroyed and the size of crystal grains increases. As a result, the tensile strength of the final cold rolled product may be lowered to less than 500 MPa. Also, when the amount of carbon (C) exceeds 0.2%, carbides of materials may increase excessively to deteriorate the elongation of the final product, and the carbides may fall off to deteriorate surface quality and corrosion resistance.

Nitrogen (N): 0.005% to 0.05%

An amount of nitrogen (N) may be, by weight %, 0.005% to 0.05%. When the amount of nitrogen (N) is less than 0.005%, a refining time may increase and the lifecycle of refractories may be reduced, resulting in an increase of manufacturing cost. Also, an equiaxed structure ratio of a slab may be lowered due to a low degree of subcooling upon casting. Meanwhile, when the amount of nitrogen (N) exceeds 0.05%, there is a high possibility that pinholes are made due to nitrogen during slab casting, the number of Cr₂N precipitates per unit area in the final cold rolled product may increase, and accordingly, a Cr depleted zone formed around the Cr₂N precipitates forms a large number of pits on the surface of the final cold rolled product, resulting in poor surface quality.

Manganese (Mn): 0.01% to 0.5%

An amount of manganese (Mn) may be, by weight %, 0.01% to 0.5%. When the amount of manganese (Mn) is less than 0.01%, refining cost may increase, and when the amount of manganese (Mn) exceeds 0.5%, an elongation and corrosion resistance may be lowered.

Chrome (Cr): 12.0% to 19.0%

An amount of chrome (Cr) may be, by weight %, 12.0% to 19.0%. When the amount of chrome (Cr) is less than 12.0%, corrosion resistance may deteriorate, whereas when the amount of chrome (Cr) exceeds 19.0%, an elongation may be lowered, and hot-rolling sticking defects may be generated.

Nickel (Ni): 0.01% to 0.5%

An amount of nickel (Ni) may be, by weight %, 0.01% to 0.5%. When the amount of nickel (Ni) is less than 0.01%, refining cost may increase, whereas when the amount of nickel (Ni) exceeds 0.5%, the impurities of the materials may increase, which lowers an elongation.

Copper (Cu): 0.3% to 1.5%

An amount of copper (Cu) may be, by weight %, 0.3% to 1.5%. When the amount of copper (Cu) is less than 0.3%, the critical current density I_(crit) may exceed 10 mA in a 5% sulfuric acid atmosphere so that sufficient acid resistance may not be secured. When the amount of copper (Cu) exceeds 1.5%, material cost may increase significantly, and furthermore, the hot workability and the elongation of the final product may be lowered.

In order to obtain a desired tensile strength in a final cold-rolled product of a ferritic stainless steel, it is necessary to secure a large number of fine carbides, and refining of crystal grains is required.

In the ferritic stainless steel having the excellent strength and acid resistance, according to an embodiment of the present disclosure, the number of carbides having a diameter of 100 nm or more per unit area may be 50 ea/100 μm².

For example, the carbides may be M₂₃C₆ type carbide-based metal precipitates.

In order to increase the number of carbides per unit area, deformed structures may need to be sufficiently formed in a hot rolled material during a hot rolling process. When the deformed structures are not sufficiently formed, it is difficult to increase the amount of carbides because carbide precipitation sites are not sufficient.

In order to sufficiently form deformed structures in the hot rolled material, a slab reheating temperature, a rough rolling reduction ratio and a hot rolled coil coiling temperature may need to be controlled during a hot rolling process, and details thereof will be described later.

That is, through the control of hot rolling process conditions, the number of carbides having a diameter of 100 nm or more per unit area can reach 50 ea/100 μm² or more. By securing a large number of fine carbides, a tensile strength of 520 MPa or more can be secured. When the above-mentioned process conditions are not satisfied, a sufficient amount of carbides cannot be obtained because of the generation of coarse carbides.

For example, when the number of carbides having a diameter of 100 nm or more is less than 50 ea/100 μm², coarsening may occur due to the small amount of carbides, which lowers the tensile strength.

For example, the ferritic stainless steel may have an average crystal grain diameter of 10 μm or less.

For example, the ferritic stainless steel according to an embodiment of the present disclosure may have a tensile strength of 520 MPa or more.

For example, the ferritic stainless steel according to an embodiment of the present disclosure may have an elongation of 20% or more.

For example, the ferritic stainless steel according to an embodiment of the present disclosure may have critical current density I_(crit) of 10 mA or less in a 5% sulfuric acid atmosphere.

A method of manufacturing a ferritic stainless steel, according to an embodiment of the present disclosure, for manufacturing the ferritic stainless steel according to an embodiment of the present disclosure, may include hot-rolling and cold-rolling a ferritic stainless steel slab including, by weight %, 0.1% to 0.2% of carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome (Cr), 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe) and other inevitable impurities, wherein a value of Equation (1) during the hot rolling satisfies 1,000 or less:

15*RHT/R4+CT  Equation (1),

where, RHT (° C.) represents a slab reheating temperature, R4(%) represents a reduction ratio of a R4 stand of rough rolling, and CT (° C.) represents a coiling temperature.

The ferritic stainless steel slab may be produced through continuous casting of molten steel containing the above-mentioned components. Thereafter, the slab may be hot-rolled and a hot-rolled coil having a thickness of 2 mm to 10 mm may be produced through hot rolling.

For example, the slab reheating temperature (RHT) may be less than 1,250° C., the reduction ratio of the R4 stand of the rough rolling may be 40% or more, and the coiling temperature (CT) may be less than 650° C. In this case, the hot rolling conditions may be set such that the value of Equation (1) satisfies 1,000 or less.

FIG. 1 is a graph for explaining a correlation between hot rolling conditions of a ferritic stainless steel and the number of carbides of a cold-rolled steel plate.

Referring to FIG. 1, it is seen that when a value of Equation (1) is 1,000 or less, the number of carbides having a diameter of 100 nm or more is 50 ea/100 μm² or more.

When the hot rolling condition of Equation (1) is not satisfied although a carbon content is sufficient, deformed structures are not sufficiently formed in the hot rolled material so that carbide precipitation sites are not sufficiently formed.

Particularly, when the coiling temperature is as high as 650° C. or higher, coarsening of precipitates occurs, and a desired number of carbides may not be obtained. As a result, crystal grains become coarse, and a desired tensile strength may not be obtained in the final product.

For example, the value of Equation (1) may satisfy 800 to 1,000.

When the value of Equation (1) is less than 800, a temperature during hot rolling may be too low, resulting in a poor plate shape.

The hot-rolled plate is subjected to an annealing process, and carbides are sufficiently precipitated through annealing at 700° C. to 900° C. in the annealing process. For example, the annealing heat treatment may be performed by a BAF annealing process. After the annealing heat treatment, a cold rolled plate having a thickness of less than 2 mm is produced through cold rolling, and final heat treatment may be performed through heat treatment at a temperature of 800° C. to 900° C.

For example, in the cold rolled plate, the number of carbides having a diameter of 100 nm or more per unit area may be 50 ea/100 μm² or more and an average crystal grain diameter may be 10 μm or less.

Hereinafter, the present disclosure will be described in more detail through embodiments.

EMBODIMENTS

Slabs of inventive steels 1 to 4 and comparative steels 1 to 9 satisfying components of Table 1 were produced through continuous casting and reheated according to hot rolling conditions of Table 2, and then a hot-rolled coil of 5 mmt was produced through hot rolling. Then, annealing heat treatment was performed at 900° C. in a BAF annealing process. Thereafter, a cold rolled steel plate having a thickness of 1 mmt was prepared by cold rolling, heat treatment was conducted at 900° C., and a final product was produced by surface short ball treatment and pickling with sulfuric acid and hydrogen peroxide.

TABLE 1 C N Mn Cr Ni Cu Inventive Steel 1 0.103 0.014 0.13 14.3 0.11 0.67 Inventive Steel 2 0.171 0.016 0.11 17.2 0.09 0.45 Inventive Steel 3 0.122 0.006 0.24 16.7 0.13 1.21 Inventive Steel 4 0.125 0.008 0.19 16.5 0.12 1.05 Comparative Steel 1 0.133 0.012 0.23 17.5 0.15 1.79 Comparative Steel 2 0.147 0.015 0.24 16.9 0.17 0.14 Comparative Steel 3 0.227 0.022 0.15 17.1 0.21 0.84 Comparative Steel 4 0.232 0.017 0.14 17.6 0.11 0.66 Comparative Steel 5 0.042 0.046 0.21 16.2 0.11 0.12 Comparative Steel 6 0.051 0.042 0.15 15.2 0.13 0.23 Comparative Steel 7 0.047 0.041 0.17 16.9 0.14 0.77 Comparative Steel 8 0.062 0.015 0.16 17.3 0.13 0.81 Comparative Steel 9 0.085 0.015 0.25 18.1 0.15 0.67

TABLE 2 RHT Steel (° C.) CT (° C.) R4 (%) 15 * RHT/R4 + CT Embodiment 1 Inventive Steel 1 1,130 550 45 927 Embodiment 2 Inventive Steel 2 1,130 550 45 927 Embodiment 3 Inventive Steel 3 1,180 550 45 943 Embodiment 4 Inventive Steel 4 1,180 580 45 973 Comparative Inventive Steel 1 1,250 550 30 1,175 Example 1 Comparative Inventive Steel 2 1,180 650 30 1,240 Example 2 Comparative Inventive Steel 3 1,180 580 30 1,170 Example 3 Comparative Inventive Steel 4 1,250 550 40 1,019 Example 4 Comparative Comparative 1,130 550 45 927 Example 5 Steel 1 Comparative Comparative 1,130 580 45 957 Example 6 Steel 2 Comparative Comparative 1,180 550 45 943 Example 7 Steel 3 Comparative Comparative 1,180 580 45 973 Example 8 Steel 4 Comparative Comparative 1,180 650 30 1,240 Example 9 Steel 5 Comparative Comparative 1,130 550 45 927 Example 10 Steel 6 Comparative Comparative 1,180 550 45 943 Example 11 Steel 7 Comparative Comparative 1,180 650 45 1,043 Example 12 Steel 8 Comparative Comparative 1,250 650 30 1,275 Example 13 Steel 9

With respect to the final cold-rolled steel plate, the number of carbides having a diameter of 100 nm or more per unit area, an average crystal grain diameter, a tensile strength, an elongation, and critical current density in a 5% sulfuric acid atmosphere were measured and shown in Table 3 below.

A TEM replica for the final cold rolled plate was made, and the number of carbide precipitates per unit area (100 μm²) was measured.

TABLE 3 Critical Current Density in a 5% The number of Average crystal Sulfuric Acid Carbides Grain Diameter Tensile Strength Atmosphere (ea/100 μm²) (μm) Elongation (%) (MPa) (mA) Embodiment 1 91 6.8 25.7 529 5.6 Embodiment 2 124 5.9 23.9 531 7.2 Embodiment 3 72 7.4 25.3 542 3.7 Embodiment 4 75 8.1 26.7 537 3.5 Comparative 32 10.8 27.6 498 8.5 Example 1 Comparative 45 11.6 25.3 508 5.9 Example 2 Comparative 37 10.5 26.7 515 4.2 Example 3 Comparative 41 12.1 27.2 498 4.5 Example 4 Comparative 81 6.5 18.8 554 3.1 Example 5 Comparative 107 5.4 25.7 527 14.5 Example 6 Comparative 227 4.8 18.5 567 6.9 Example 7 Comparative 305 5.3 19.2 572 7.5 Example 8 Comparative 13 18.1 30.1 457 15.6 Example 9 Comparative 19 17.3 29.8 453 13.8 Example 10 Comparative 28 20.6 28.9 476 6.3 Example 11 Comparative 21 14.1 27.7 481 5.4 Example 12 Comparative 26 12.0 26.3 491 4.2 Example 13

FIG. 2 is a picture showing a distribution of precipitates in a ferritic stainless steel cold-rolled steel plate according to an embodiment of the present disclosure, taken by a transmission electron microscope (TEM). FIG. 3 is a picture showing a distribution of precipitates in a ferritic stainless steel cold-rolled steel plate according to a comparative example of the present disclosure, taken by a TEM.

FIG. 2 is a picture showing a cold-rolled steel plate according to Embodiment 2, and FIG. 3 is a picture showing a cold-rolled steel plate according to Comparative Example 2.

Referring to FIGS. 2 and 3, a value of 15*RHT/R4+CT according to the relational expression relating to a slab reheating temperature, a R4 reduction ratio, and a coiling temperature upon hot rolling exceeds 1,000, as in Comparative Examples 1 to 4, so that sufficient deformed structures for a hot rolling material are not formed and thus carbide precipitation sites are not sufficient although a carbon content is sufficient.

Furthermore, when the coiling temperature is high as in Comparative Example 2, coarsening of precipitates occurs so that a desired number of carbides may not be obtained.

When the copper content is excessive as in Comparative Example 5, the elongation of the final product becomes 18.8%, which means that the elongation deteriorates. When the copper content is small as in Comparative Example 6, the critical current density I_(crit) is 14.5 mA so that sufficient acid resistance may not be secured.

When the carbon content is excessive as in Comparative Examples 7 and 8, the number of carbides increases and an elongation decreases. When the carbon content is small as in Comparative Examples 9 to 13, it is confirmed that the crystal grain size increases and the tensile strength is lowered to less than 500 MPa.

FIG. 4 is a graph for explaining a correlation between the number of carbides and the tensile strength of a cold-rolled steel plate made of a ferritic stainless steel.

FIG. 4 is a graph showing the numbers of carbides and tensile strengths of the cold-rolled steel plates according to the embodiments and the comparative examples. Referring to FIG. 4, it is confirmed that as the number of carbides increases, the tensile strength tends to increase accordingly.

While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure. 

1. A ferritic stainless steel having an excellent strength and acid resistance, comprising, by weight %, 0.1% to 0.2% of carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome (Cr), 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe) and other inevitable impurities, wherein a number of carbides having a diameter of 100 nm or more per unit area is 50 ea/100 μm² to 200 ea/100 μm².
 2. The ferritic stainless steel of claim 1, wherein an average crystal grain diameter is 10 μm or less.
 3. The ferritic stainless steel of claim 1, wherein a tensile strength is 520 MPa or more.
 4. The ferritic stainless steel of claim 1, wherein an elongation is 20% or more.
 5. The ferritic stainless steel of claim 1, wherein a critical current density I_(crit) in a 5% sulfuric acid atmosphere is 10 mA or less.
 6. A method of manufacturing a ferritic stainless steel having an excellent strength and acid resistance, comprising hot-rolling and cold-rolling a ferritic stainless steel slab comprising, by weight %, 0.1% to 0.2% of carbon (C), 0.005% to 0.05% of nitrogen (N), 0.01% to 0.5% of manganese (Mn), 12.0% to 19.0% of chrome (Cr), 0.01% to 0.5% of nickel (Ni), 0.3% to 1.5% of copper (Cu), the remainder iron (Fe) and other inevitable impurities, wherein a value of Equation (1) during hot rolling satisfies 1,000 or less, Equation (1) being 15*RHT/R4+CT, where RHT (° C.) represents a slab reheating temperature, R4(%) represents a reduction ratio of a R4 stand of rough rolling, and CT (° C.) represents a coiling temperature.
 7. The method according to claim 6, wherein the value of Equation (1) satisfies 800 to 1,000.
 8. The method according to claim 6, wherein the RHT is below 1,250° C., R4 is above 40%, and CT is below 650° C.
 9. The method according to claim 6, wherein a number of carbides having a diameter of 100 nm or more per unit area of a cold-rolled plate is 50 ea/100 μm² to 200 ea/100 μm², and an average crystal grain diameter of the cold-rolled plate is 10 μm or less. 